Spunbonded fibrous sheets made of multiple plexifilamentary strands of oriented polyethylene film fibrils are disclosed in U.S. Pat. No. 3,169,899 (Steuber). Such sheets are produced commercially by E. I. du Pont de Nemours and Company under the trademark "Tyvek.RTM." spunbonded olefin. The sheets have proven useful in diverse applications which take advantage of the sheets' unusually good combination of strength, tear resistance and permeability properties.
Polyethylene pulps can be prepared by cutting these Tyvek.RTM. sheets into small pieces and beating the cut pieces in an aqueous refiner. Examples of other methods for producing polyolefin pulps are given in Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 19, 3rd edition, John Wiley & Sons, pp. 420-435 (1982). This reference describes synthetic pulps as generally being very fine, highly branched, discontinuous, water-dispersible fibers made of plastics. Methods are described for producing synthetic pulps by solution flash-spinning, emulsion flash-spinning, melt-extrusion/fibrillation and shear precipitation. The pulps may be blended with other fibers in an attempt to make papers, sheets or boards by conventional wet-lay papermaking techniques. Such pulps are also identified as being used as bonding agents for certain nonwoven materials such as dry-laid, Rando-Webber formed sheets and wet-laid, Fourdrinier-formed sheets.
U.S. Pat. No. 4,608,089 (Gale et al.) discloses forming oriented polyethylene film-fibril pulps by cutting a flash-spun polyethylene sheet (e.g., Tyvek.RTM.) into pieces, forming an aqueous slurry with the pieces and then refining the pieces with disc refiners to form a pulp that is particularly suited for cement reinforcement. The pulp is prepared from flash-spun plexifilaments which are cut into small pieces and beaten in an aqueous medium. Although these pulps have found some utility in reinforcing cement composites, they are not useful in making high grade polyethylene paper.
European Patent Application No. 292,285 (Gale et al.) discloses forming improved oriented polyethylene film-fibril pulps for reinforcing various articles. The pulps are prepared from flash-spun, oriented, linear polyethylene, plexifilamentary strands that are converted into small fibrous pieces that are then reduced in size by refining in an aqueous medium to form a fibrous pulp slurry. The pulp slurry is then further refined until an average fibrid length of no greater than 1.2 mm is achieved and no more than 25% of the fibrous pulp is retained on a 14-mesh screen and at least 50% of the pulp passes through the 14-mesh screen but is retained by a 100-mesh screen. Various articles are disclosed which can be made from the improved pulp. These include, speciality synthetic papers, reinforced gaskets, reinforced cements, reinforced resinous articles and heat-bonded sheets which are particularly useful for filtration applications. Although these pulps have found some utility in reinforcing applications and in producing paper hand sheets, they are not satisfactory for making high grade, low basis weight polyethylene paper on conventional continuous wet-lay paper-making equipment.
One of the problems encountered when trying to make high grade paper on conventional continuous paper-making equipment with these types of polyethylene fibers is that they tend to stick to the drying cans while the sheet is being dried. Moreover, during the drying process the sheet will stretch in the machine direction and lose tension in between the drying cans. This causes the paper sheet to have poor uniformity.
Although there are some methods available which allow synthetic paper to be made from polyethylene pulp on conventional paper-making eqipment, they require unique fibers and process steps. One such example is disclosed in U.S. Pat. No. 4,783,507, where the inventive feature rests in the use of two polyethylene pulps, one that melts at 95.degree. C. or below and one that melts at higher temperatures. Paper can be prepared from the two polyethylene pulps on a conventional paper-making machine using drying cans which are heated by 212.degree. F. steam. The polyethylene pulps used to make the paper are prepared by the process of U.S. Pat. No. 3,920,508 (Yonemori). Yonemori discloses flash-spinning an emulsion of polyethylene and refining the resulting fibers.
Clearly, what is needed is a process for producing high grade polyethylene paper from pulp on conventional continuous wet-lay paper-making equipment. The paper should have reduced elongation, high strength and a low number of defects (i.e., increased uniformity). Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the drawings and the detailed description of the invention which hereinafter follows.